Conventional Q-switched fiber lasers operating at multi-longitudinal-mode oscillation usually suffer from self-mode-locking-induced temporal instability, relatively strong noise, and low coherence. Here, we address the challenge through demonstrating, for the first time, to the best of our knowledge, a single-longitudinal-mode (SLM) Er-doped fiber (EDF) laser passively Q-switched by a few-layer Bi2Se3 saturable absorber (SA). The Bi2Se3 SA prepared by the liquid-phase exfoliation method shows a modulation depth of ～5% and saturation optical intensity of 1.8 MW/cm2. A section of 1-m unpumped EDF together with a 0.06-nm-bandwidth fiber Bragg grating is used as an ultra-narrow autotracking filter to realize SLM oscillation. Stable SLM Q-switching operation at 1.55 μm is successfully achieved with the spectral linewidth as narrow as 212 kHz and the pulse duration of 2.54 μs, manifesting near-transform-limited pulses with a time-bandwidth product of 0.53. In particular, we found that the SLM Q-switching possesses the higher signal-to-noise ratios of 62 dB (optical) and 48 dB (radio frequency), exhibiting its advantages of low noise and high stability. Such an SLM Q-switched fiber laser could gain great interest for some applications in coherent detection, coherent optical communications, and high-sensitivity optical sensing.

On-chip spectroscopic sensors have attracted increasing attention for portable and field-deployable chemical detection applications. So far, these sensors largely rely on benchtop tunable lasers for spectroscopic interrogation. Large footprint and mechanical fragility of the sources, however, preclude compact sensing system integration. In this paper, we address the challenge through demonstrating, for the first time to our knowledge, a supercontinuum source integrated on-chip spectroscopic sensor, where we leverage nonlinear Ge22Sb18Se60 chalcogenide glass waveguides as a unified platform for both broadband supercontinuum generation and chemical detection. A home-built, palm-sized femtosecond laser centering at 1560 nm wavelength was used as the pumping source. Sensing capability of the system was validated through quantifying the optical absorption of chloroform solutions at 1695 nm. This work represents an important step towards realizing a miniaturized spectroscopic sensing system based on photonic chips.

In this paper, both nonlinear saturable absorption and two-photon absorption (TPA) of few-layer molybdenum diselenide (MoSe₂) were observed at 1.56 μm wavelength and further applied to mode-locked ultrafast fiber laser for the first time to our knowledge. Few-layer MoSe₂ nanosheets were prepared by liquid-phase exfoliation method and characterized by x ray diffractometer, Raman spectroscopy, and atomic force microscopy. The obtained few-layer MoSe₂ dispersion is further composited with a polymer material for convenient fabrication of MoSe₂ thin films. Then, we investigated the nonlinear optical (NLO) absorption property of the few-layer MoSe₂ film using a balanced twin-detector measurement technique. Both the saturable absorption and TPA effects of the few-layer MoSe₂ film were found by increasing the input optical intensity. The saturable absorption shows a modulation depth of 0.63% and a low nonsaturable loss of ～3.5%, corresponding to the relative modulation depth of 18%. The TPA effect occurred when the input optical intensity exceeds ～260 MW/cm2. Furthermore, we experimentally exploit the saturable absorption of few-layer MoSe₂ film to mode lock an all-fiber erbium-doped fiber laser. Stable soliton mode locking at 1558 nm center wavelength is achieved with pulse duration of 1.45 ps. It was also observed that the TPA process suppresses the mode-locking operation in the case of higher optical intensity. Our results indicate that layered MoSe₂, as another two-dimensional nanomaterial, can provide excellent NLO properties (e.g., saturable absorption and TPA) for potential applications in ultrashort pulse generation and optical limiting.